I think the easy availability of chronographs really busted the bubble on wildcatters.
The golden age of wildcats was in the 50’s/60’s. If you read vintage publications shooters were making velocity estimates based on things like bullet drop over 200/300 yards. Wildcatters could make outrageous velocity claims on shoulder angles, (Weatherby anyone?) straight cases, case capacity because no one had chronographs to check their claims.
If they did get high velocities, it was due to extremely high pressures. P. O Ackley comes to mind with his claims of velocity increases with his blown out cases. He just did not get a little more velocity, he got a lot more, and it was due to extremely high pressures.
Below are comparisons of Ackley's published data compared to pressure tested data.
49th edition of Lyman Handbook, the max load of a standard 30-30 with a 150 grain bullet and using 28 grs IMR 3031, the velocity is 2145 with a pressure of 38,000 cup.
In Ackley’s own handbook, the maximum load for a 30-30 AI for a 150 grain bullet using IMR 3031 is 38 grains for a velocity of 2700 fps.
From web data, the case capacity of the 30-30 Ackley vs the unImproved Winchester parent differ by 5% http://www.gmdr.com/lever/3030atext.htm
yet here you have Ackley stuffing in 10 additional grains of powder and claiming a velocity increase of 125% over the standard 30-30.
The only way to get those sort of velocities through incredibly high pressures.
If you go to your 1957 Gun Digest, factory ballistics for the Winchester 180 grain Super Speed 30-06 is 2700 fps. Modern reloading data shows you can push a 180 Barnes with 55.7 grs IMR 4350 to 2685 fps, in close agreement with older factory data.
Ackley’s handbook gives reloading data of 61 grains IMR 4350 with a 180 grain bullet for a velocity of 3053 fps.
Noslers shows a max load for the 30-06AI of 56.5 grs IMR 4350 with a 180 gr bullet at 2835.
For the 30-06AI Ackley is putting 5.3 additional grains of powder in the case and claiming a velocity increase of 113% over the parent cartridge. His data is pushing bullets 218 fps faster than modern pressure tested ammunition of the same case.
The only way to do this is through incredibly high pressures.
One of these days we will have cheap pressure testing equipment (or I hope we will have cheap pressure testing equipment) and even more of the outrageous claims that have carried down from the wildcatting 50’s/60’s can be tested, by enough people, and found false.